On the nature of the companion to Van Biesbroeck 8

Abstract

We present here the results of the first numerical evolutionary calculations for very low-mass stars (masses in the range of 0.01–0.1 M⊙) with ages up to the age of the Galaxy. Our calculations shed new light on the nature of the recently discovered object VB 8B (ref. 1), the companion to Van Biesbroeck's star no. 8 (VB8; ref. 2), and support the identification¹ of this object as a low-mass star in which hydrogen burning was never able to establish thermal equilibrium. For plausible ages of this binary stellar system (≳10⁹ yr), we estimate the mass, M, of VB 8B to be in the range 0.04–0.08 M⊙ (see also ref. 1 and refs therein). The object is supported primarily by degenerate electron pressure, and we find that its present values of central density and temperature are ρ_c ≃ 500–2,000 g cm⁻³ and T_c ≃ 1.0−1.5 × 10⁶ K, respectively. The star VB8 itself^3–5 may also be a low-mass star which marginally failed to establish main-sequence hydrogen burning. Thus, the mass ratio for this binary system is probably not greatly different from unity, and we predict an orbital period of ≳40yr. Our evolutionary tracks are also applicable to the interpretation of observations of other very low-luminosity dwarfs (see refs 3, 5–7). The study of such objects may be directly relevant to the development of an understanding of the ‘dark matter’ that has been inferred, from dynamical considerations, to be present in our Galaxy and many extragalactic systems (see, for example, refs 8, 9).